e, that millions are produced. These living organisms cause gases to
form, and they continue to breed and grow and multiply so long as they
have anything to feed on."

"And we eat all that stuff and call it good?"

"Yes; and why not? Every part of your body contains the little
creatures, and they really keep you alive, and preserve your health, as
well as prevent diseases."

"Why so?"

"Most germs are of the harmless type, and it is because of the vast
numbers of the harmless ones that the few poisonous or disease germs are
killed. Water has millions of them in every cubic inch. Professor Dewar,
a great English chemist, calls them nature's policemen. If a typhoid
fever germ, for example, should be introduced among so many germs, as is
the case every day, a fight at once takes place, and where a person is
finally attacked with the fever, it is because the germs escaped the
policemen who were on duty."

"That sounds like a romance."

"Yes; the life history of those germs is really a wonderful thing, and
books have been written about them. They exist in tribes, as it were;
some of them can live only where oxygen is present, and some live on
nitrogen only; others on carbon. But that is not all. Man has learned to
use them, so they will work just as surely as our yaks work for us under
our direction."

"How interesting! In what way do we use them?"

"In what is called the septic system of treating sewage. You know that
sewage from the kitchen contains all kinds of meat and vegetables, and
the more it has fermented the stronger becomes the odor and the greater
are the number of bacteria in the sewage. The sewage in the liquid state
is first placed in a reservoir, and at a certain temperature the germs
grow very rapidly, and, of course, eat up the vegetable and animal
matter until it is nearly all consumed. Then it is run off into another
reservoir which has another tribe of germs in it, those that live on
carbon, and which are not harmful to man, and when these two tribes meet
war is declared, and they fight to the death. The harmless germs are
victorious in every battle, and when the sewage is discharged into a
stream, or used for irrigating purposes, few, if any, of the harmful
germs remain."

"So in using germs the object is to cultivate one kind to kill another
kind?"

"Not always; chemists have found out that man and animals absorb oxygen
and expel nitrogen, in order to live; and that plants take in and live